1. Field of the Invention
The present invention relates to an optical device including an integrated optical coupler for effecting light branching and/or combining by splitting the wavefront of a propagated light wave for use in opto-electrical and optical integrated circuits, compound-resonators and the like which are needed in the field of optical communication and the like. The present invention also relates to a method for forming an optical coupler in a waveguide of an optical device.
2. Related Background Art
In recent years, there has been a rapid growth in the importance of opto-electrical integrated circuits when optical and electrical devices are integrated on a common substrate. In the field of optical communication, there is a need to integrate a plurality of devices having different structures. However, the present process technology thereof has not yet satisfied the design needs in that field. For example, a directional coupler type optical coupler or a branching and/or combining device has a structure that can be readily designed, but the length of this device is more than 1 mm. Therefore, the degree of integration cannot be increased, and at the same time the process accuracy becomes a problem.
On the other hand, as an example that is suitable for the integrated structure, there has been proposed a beam splitter type branching and/or combining device as is disclosed in the "Journal of Light Wave Technology", Vol. 16, No. 6, pp. 837-846, 1988. FIG. 1 shows a plan view of this type of beam splitter 64. In FIG. 1, there are formed an input port 61 and output ports 62 and 63. The power of the light entering the input port 61 is branched and propagated through the two output ports 62 and 63 by the beam splitter 64. The device shown in FIG. 1 is fabricated as follows.
FIG. 2 is a cross-sectional view of the port or waveguide portion in FIG. 1. After an n-type A1.sub.0.1 Ga.sub.0.9 As layer 72 is grown on an n-type GaAs substrate 71 by the metal organic vapor phase epitaxy (MOVPE) method, a groove 77 is formed for controlling a lateral mode of the waveguide portion. An n-type GaAs core layer 73 is then buried flatly by the chloride vapor phase epitaxy method, and an n-type A1.sub.0.5 Ga.sub.0.5 As layer 74, an n-type A1.sub.0.1 Ga.sub.0.9 As layer 75 and a p-type GaAs layer 76 are grown thereon by the MOVPE method.
The structure of the beam splitter 64 is produced by perpendicularly forming a groove in the GaAs core layer 73. The groove 64 forms an angle of 45 degrees relative to the waveguide, and its depth is stopped halfway in the GaAs core layer 73. At the beam splitter 64, the beam propagated through the input port 61 is branched by the depth of the perpendicular groove 64 with respect to a straightforward direction or a direction toward the port 63, while the beam is branched by the surface of the groove 64 functioning as a total reflection mirror with respect to a right-angle direction or a direction toward the port 62.
In this prior art device, however, sufficiently satisfactory characteristics could not be obtained because the groove 64 is formed by a normal reactive ion etching (RIE) method. As is also described in the above-mentioned article, the fabrication accuracy of the RIE method cannot satisfy the requirements of the design of groove 64. In order to increase the fabrication accuracy, the focused ion beam etching (FIBE) method may preferably be used. However, the FIBE method has considerable drawbacks, and this method is not utilized widely.
The first problem of a fabrication method using the FIBE method is that damage will be generated in an etched surface, especially, in the core portion of a waveguide. This results in sources of waveguide losses and degradation of the waveguide. Normally, the damage of dry etching such as the FIBE method, can be optically understood as the decrease of a band gap in an energy band structure of the waveguide. Therefore, influences become great when an optical coupler such as the above-discussed groove 64 shares the waveguide with a waveguide having a gain for propagated light, such as waveguides containing an optical amplifier and an active filter, for example.
The second problem with using the FIBE method is in that its throughput is low and its yield and reproducibility are poor. The process accuracy of a branching and/or combining portion should be sufficiently high since the manner of branching and combining is determined by the relationship of the branching and/or combining portion with a field profile or distribution of a light wave propagated through the waveguide. In the case of an integrated coupler/splitter, this becomes a serious problem.